Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades are the primary elements for converting wind energy into electrical energy. The blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between its sides. Consequently, a lift force, which is directed from the pressure side towards the suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
At wind speeds well below the rated wind speed of a wind turbine, the pitch angle of the rotor blades is typically maintained at the power position in order to capture the maximum amount of energy from the wind. However, as wind speeds reach and exceed the rated wind speed, the pitch angle is adjusted towards feather to maintain the power output of the wind turbine at its rated power. As a result, the power output of the wind turbine is constrained at wind speeds above the rated wind speed. In addition, the aerodynamic loads acting on the rotor blades continually increase with increasing wind speeds while the pitch angle of the rotor blades is maintained at the power position and then begin to decrease as the pitch angle is adjusted towards feather with wind speeds above the rated wind speed. Such control of the wind turbine typically creates a peak in the aerodynamic loading on a wind turbine at its rated wind speed. For example, FIG. 1 illustrates a graph of wind speed (x-axis) versus loads (y-axis) for a typical wind turbine. As shown, aerodynamic loads on the wind turbine increase along loading line 10 to a peak 12 at the rated wind speed (indicated by line 14) and then decrease as the rotor blades are pitched toward feather in order to maintain the wind turbine at its rated power.
To prevent the formation of such a peak 10, peak-shaving control methods are known that are used to reduce the loads on a wind turbine at or near the rated wind speed. In particular, these control methods typically begin to adjust the pitch angle of the rotor blades at some point prior to the rated wind speed. For example, as shown in FIG. 1, by adjusting the pitch angle of the rotor blades towards feather prior to reaching the rated wind speed (e.g., at a given wind speed threshold indicated by line 16), the loads acting on the rotor blade at or near the rated wind speed may be reduced along loading curve 18. Specifically, as shown in FIG. 1, the use of a peak shaving control method may create a peak shaving range 20 at which loads are reduced along a range of wind speed values.
However, such a control method also results in a reduction in the overall efficiency of the wind turbine, as power production at or near the rated wind speed is sacrificed (i.e., by prematurely pitching the rotor blades) in order to reduce blade loading. For example, FIG. 2 illustrates a graph of wind speed (x-axis) versus power (y-axis) for wind turbines implementing the control methodologies described above with reference to FIG. 1. Specifically, power curve 110 defines the baseline power output for a wind turbine using a control methodology in which the rotor blades are pitched towards feather at or near the rated wind speed (i.e., the power output corresponding to the loading curve 10 of FIG. 1) and power curve 120 defines the baseline power output for a wind turbine using a peak-shaving control method (i.e., the power output corresponding to the loading curve 20 of FIG. 1). As shown, due to the peak-shaving control methodology, the power curve 120 transitions from a positive curvature to a negative curvature at an inflection point 122 defined at the wind speed threshold 16 (i.e., the point at which the pitch angle adjustment is initiated), thereby indicating that the power production has been curtailed at wind speeds above this threshold 16.
Typically, the average wind turbine site experiences wind speeds below the rated wind speed for the majority of the operating life of the wind turbine(s) located at such site. For example, in some instances, a wind turbine site may experience wind speeds at or above the rated wind speed only 6% of the time. As such, the reduction in power output resulting from the use of peak-shaving control methods at wind speeds at and below the rated wind speed can be quite significant, particularly for a wind turbine site including a large number of wind turbines. Moreover, conventional control methodologies that maintain the power output of a wind turbine as its rated power at increased wind speeds may significantly limit the control options for controlling an output-constrained wind farm.
Accordingly, a system and method for controlling the wind turbines of a wind farm that allows for the total power production of the farm to be increased across a wide range of wind speeds would be welcomed in the technology.